In continuously casting a round billet having a round shape cross section, compared with the continuous casting of a rectangular billet having a rectangular shape cross section, the billet is unevenly cooled because a mold inner wall (an inner peripheral surface in the case of the round billet mold) is unstably in contact with the billet. When the billet is excessively unevenly cooled, a longitudinal cracking defect is generated in the billet, and a break out is generated due to the longitudinal cracking defect. Therefore, the casting cannot be continued at last.
In order to circumvent the generation of such situation, there have been proposed such various methods that an inner diameter of the mold is decreased according to solidification shrinking, and mold powder fed into the mold are improved during the continuous casting to adjust contact between the mold inner peripheral surface and the billet. For example, Japanese Utility Model Application Publication No. 59 (1984)-165748 proposes a mold, in which an inner diameter is decreased downward and a decrease ratio of the inner diameter is changed in two steps. Further, Japanese Utility Model Application Publication No. 59 (1984)-165749 proposes a mold, in which a tapered surface whose inner diameter is continuously decreased downward and the change in inner diameter is matched with the solidification shrinking. According to the proposed molds, it is said that the uniform contact can be achieved between the mold inner peripheral surface and the billet.
However, in the mold proposed in Japanese Utility Model Application Publication No. 59 (1984)-165748 mentioned above, it is difficult to maintain the good contact between the mold inner peripheral surfrace and the billet in a whole region from an upper portion to a lower portion of the mold during the continuous casting. Moreover, in the mold proposed in Japanese Utility Model Application Publication No. 59 (1984)-165749 mentioned above, there is a problem in application, although it is possible theoretically that the good contact is maintained between the mold inner peripheral surfrace and the billet in the whole region from the upper portion to the lower portion of the mold during the continuous casting. That is, a solidification shrinking amount of the billet is difficult to be measured, and it is necessary to change the mold according to each steel grade because the solidification shrinking amount is changed when a chemical composition of steel to be cast is changed, and further, the shrinking amount of casting direction is changed when a casting speed is changed. Accordingly, such proposed molds cannot be used in a commercial operation.
The applicant has proposed a mold in Japanese Patent No. 3022211, in which the uniform contact is achieved between the mold inner peripheral surface and the billet to perform uniform cooling in continuously casting the round billet. The mold from the upper edge to the lower edge is divided into at least three regions along the casting direction, and the inner diameter of the mold is gradually decreased from the upper edge toward the lower edge by defining a rate of change in mold inner diameter per unit length along the casting direction in each region.